The subject invention relates to high power diffusion cooled carbon dioxide slab lasers. An example of such a laser can be found in commonly owned U.S. Pat. No. 5,140,606 incorporated herein by reference. These lasers include a pair of rectangular metal electrodes mounted within a sealed housing containing the laser gas. The electrodes are spaced closely together to define a slab shaped discharge region. RF power is used to excite the gas to generate laser light.
These types of lasers typically use a hybrid optical resonator which has an unstable resonator in the width dimension of the parallel facing electrodes and a waveguide type resonator in the vertical (i.e., the “gap”) dimension separating the two electrodes. Early designs used a positive branch unstable resonator (see, for example, U.S. Pat. No. 4,719,639, incorporated herein by reference). Later designs have used a negative branch unstable resonator (see U.S. Pat. No. 5,048,048).
The positive branch unstable resonator designs are about an order of magnitude more difficult to align than the negative branch designs but the designs are much less sensitive to output beam pointing variations as a function of changes in the curvature of the resonators mirrors with temperature changes. On the other hand the negative branch resonators are easier to align but their beam pointing variations are much more sensitive to mirror curvature changes with temperature. The variation in the mirror curvature with temperature causes the pointing of the output laser beam to vary.
To obtain the easier to align advantages of the negative branch unstable resonators, the large changes in the lasers beam pointing stability with changes in mirror curvature with temperature needs to be solved. This is especially true as the discharge length becomes shorter and the width increases. Analysis indicates that the pointing variations of the laser beam increase directly with the width of the negative branch unstable resonator and inversely as the square of its length. Designing shorter industrial CO2 laser is looked upon with favor in the industrial application of CO2 lasers as long as beam quality is not compromised. In this regard, as the length of the discharge is made shorter, the width of the discharge needs to be increased to maintain the same discharge area required to obtain the same laser output power.
In a negative branch unstable resonator design, the output coupling mirror and the return mirror have concave surfaces. The mirrors normally extend over the entire width of the parallel facing electrodes that are separated by a small gap in the vertical dimension except at the output coupling side. Direct thermal heating of the mirror's reflecting surface by the laser beam occurs because the high reflecting thin films deposited on the mirror's surface have a very small but finite absorption which heats the surface of the mirror. The heat from this reflecting surface propagates through the thickness of the mirror, thereby, establishing a temperature gradient between the front and back surfaces of the mirror.
Since the back of the mirror is normally attached to a massive mechanical housing holding the mirror, the back of the mirror is usually cooler then the front surface, thus maintaining a temperature gradient between the front and back surfaces. This temperature gradient increases with laser power and causes a distortion or warping of the mirror's surface thereby disturbing the optical resonator's geometry resulting in undesirable changes in the laser's output beam performance such as in beam profile shapes and in pointing variations. In the case of a thermal gradient, the warping causes the concave mirror surface (the side towards the laser discharge) to become more convex.
Changes in output beam parameters in response to changes in operating conditions of the laser, such as RF power input to the discharge, pulse repetition frequency, duty cycle, etc., cause changes in the mirror thermal gradient and such changes are not desirable in industrial laser systems. The elimination or reduction of these laser mirror thermal distortion effects is highly desirable and is the focus of this disclosure. It is important to note that this invention has broader applications then to only reducing the pointing variations of an unstable laser resonator output beam. For example, it can have relevance to reducing mirror distortions in optical systems handling high optical power.
Our analytical analysis and experimental testing have indicated that the major cause of beam pointing variations in a slab CO2 laser's output that utilizes a negative branch unstable resonator is changes in the curvature of the surfaces of the high reflecting feedback mirror and of the output coupling mirror of the unstable resonators. This change in curvature is caused by the three thermal effects listed below:
1. A temperature gradient across the thickness of the mirror causes bending of the mirrors. Since the back of the mirror is cooler than the reflecting surface of the mirror, the mirror becomes less concave due to the fact that the front surface expands more the back surface. We have found that a few degrees temperature gradient can make the laser's output beam deflect by an unacceptable amount.
2. An increase in the average temperature of the mirror by the heating of the mirror by the laser radiation also causes the mirror to become less concave due to the thermal expansion of the mirror's material. Changes in the mirror's curvature changes the optimum alignment of the resonator and deteriorates the laser's performance. In our case, the mirror material is copper but similar changes in curvature effects can be expected with other mirror materials, such as Silicon, for example.
3. The bimetallic effect between the mirror material (usually copper) and the large mirror holder pate-form (usually made of aluminum) also changes the radius of curvature due to bending caused by the two materials having different thermal coefficient of expansion. It is important to note that the thermal coefficient of expansion of aluminum is larger than for copper. In this case, the bimetallic effect causes the concave mirror to become more concave.
The deleterious effects of mirror heating has been addressed in the prior art. For example, U.S. Pat. No. 5,020,895 describes using an adaptive optics technique to compensate for the thermal deformation of the mirrors. This approach involved the use of active electronic feedback circuitry coupled with a sensor and an actuator, such as a piezoelectric or a regulative liquid or gas pressure chamber, to provide a force to counteract the radius of curvature arising from the thermal effects on the mirror. This approach adds undesirable complexity and cost of the laser.
Another prior art technique was to use active electronic feedback circuitry coupled with a temperature sensor and a heating element to heat the back of the resonator's mirror to establish an appropriate temperature distribution to counteract the change in the radius of curvature (see U.S. Pat. No. 5,751,750). The latter patent also reported on an approach to couple out a small portion of the laser beam and use it to irradiate the back surface of the mirrors to equalize the temperature of the front and back surface of the mirrors. Both of these approaches add undesired complexity and cost to the lasers.
U.S. Pat. No. 4,287,421 discloses a transparent mirror material having reflective coatings pre-selected to allow a small amount of laser radiation to propagate through the coatings and the mirror material, to be in turn, absorbed by a coating on the back side of the mirror. The reflection and absorption parameters are selected so that radiation absorbed by the absorbing coating is sufficient to heat up the back side of the mirror to compensate for the temperature gradient between the front and back of the mirror. The limitation of this approach is the requirement for a transparent material for the mirror. Consequently, it is not useful with copper mirrors which we believe are more resistive to damage than either Si or ZnSe mirrors.